fs_mgr: remove code that gets slot_suffix from misc partition

'slot_suffix' for A/B devices *must* be provided in kernel command line
or in device tree. The removed code was unnecessarily looking it up in
the 'misc' partition.

b/33254008

Test: Boot sailfish

Change-Id: I329af2d9a98c34a941596779eb78d14337b864e5
Signed-off-by: Sandeep Patil <sspatil@google.com>